1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a transmission scheme for a Multiple Input Multiple Output (MIMO) communication system.
2. Description of the Related Art
In a MIMO communication system that transmits/receives data using a plurality of transmission antennas and reception antennas, a MIMO channel formed by the transmission and reception antennas is disassembled into a plurality of independent spatial channels. Each of the spatial channels is mapped to one dimension, and the use of additional dimensions created by the multiple transmission and reception antennas improves performance of the MIMO system.
In a wireless communication system, transmission data forms a symbol stream through a coding and modulation process, and the symbol stream is phase-shifted into a radio frequency (RF) modulated signal that is suitable to be transmitted over a wireless channel.
A data transmission scheme using the MIMO system is classified into a Spatial Multiplexing (SM) scheme and a Spatial Diversity (SD) scheme. The SM scheme simultaneously transmits different data using multiple transmission antennas, thereby transmitting data at a high rate without increasing a bandwidth of the system. The SD scheme transmits single data stream via multiple transmission antennas, thereby obtaining transmit diversity gain.
FIG. 1 is a block diagram illustrating a structure of a transmitter based on a Bell Labs LAyered Space-Time (BLAST) scheme, which is the conventional SM scheme. An input bit stream is multiplexed into four parallel bit streams S1, S2, S3 and S4 by a multiplexer (MUX) 101. The individual bit streams S1, S2, S3 and S4 are modulated into modulation symbols X1, X2, X3 and X4 according to a modulation technique of their associated modulators 103, and then transmitted through four associated transmission antennas. In this case, the modulation technique is a quadrature phase shift keying (QPSK) scheme. In the BLAST scheme, multiplexing gain is in proportion to the number of transmission antennas and reception antennas. However, the BLAST scheme, as it provides no diversity gain, there is a decrease in bit error rate (BER) performance in a high signal-to-noise ratio (SNR) environment, and it suffers an abrupt change in performance depending on the type of receiver and transmission optimization technique.
FIG. 2 is a block diagram illustrating a structure of a transmitter based on a Double Space-Time Transmit Diversity (DSTTD) scheme, which is the conventional SD scheme. Modulation symbols X1, X2, X3 and X4 modulated through the process of FIG. 1 are pre-coded by a transmission matrix of a pre-encoder 105 before being transmitted. The transmission matrix has a form defined by Equation (1):
                    B        =                  [                                                                      x                  1                                                                              x                  2                                                                              x                  5                                                                              x                  6                                                                                                      x                  2                  *                                                                              -                                      x                    1                    *                                                                                                x                  6                  *                                                                              -                                      x                    5                    *                                                                                                                        x                  3                                                                              x                  4                                                                              x                  7                                                                              x                  8                                                                                                      x                  4                  *                                                                              -                                      x                    3                    *                                                                                                x                  8                  *                                                                              -                                      x                    7                    *                                                                                ]                                    (        1        )            
The DSTTD scheme using the 4×4 transmission matrix shown in Equation 1 has both a multiplexing gain of 2 and a diversity gain of 2, thus contributing to performance improvement, but results in a decrease in data rate due to the reduction in the multiplexing gain.
For improvement in system performance, the MIMO system has recently been designed so as to support at least one of the SM techniques and the SD techniques. In this scheme, a particular transmission technique is selected in a given time period according to channel conditions and required results.
However, because a transmission technique for spatial multiplexing and a transmission technique for spatial diversity are different from each other in design, supporting both of the transmission techniques for spatial multiplexing and spatial diversity within one system causes an increase in complexity of transceivers.